Method for controlling projection of optical layup template utilizing cooperative targets

ABSTRACT

A projector system having a process utilizing three-dimensional data, thereby allowing the system to account for rotational and translational differences between the projector and the object upon which the laser light is directed. Reference targets located on the object are in a known relationship to the projected three dimensional data. The reference targets are retro-reflective, such that light steered by the projector, when impinging on the reference targets, will return to the projector for detection and determination of the relative translation and orientation between projector and object. The three-dimensional data is then converted to a format suitable for projection using said translation and orientation information.

RELATED PATENTS APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.08/437,902 filed May 9, 1995, now abandoned which is a continuation ofU.S. patent application Ser. No. 08/113,456 filed Aug. 27, 1993 now Pat.No. 5,450,147, which is a continuation-in-part of U.S. patentapplications Ser. No. 07/951,603 filed Sep. 28, 1992, now Pat. No.5,341,183, assigned to The Boeing Company.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a rapidly scanned laser system that accuratelyidentifies locations on an object, and more particularly, to a rapidlyscanning laser system utilizing a three-dimensional (3-D) data setprojected onto contoured surfaces.

2. Discussion of the Prior Art

U.S. Pat. No. 4,883,352 is directed to a galvanometer that scans flatpatterns. U.S. Pat. No. 4,883,352 teaches that in critical applications,fine focus control can be accomplished via changes of laser wavelength.The device shown uses quadrant photodetector to actively compensate forrelative lateral motion between the scanners and the site being scanned.

The present optical layup template (OLT) can be distinguished by notingthat while galvanometer scanning is known, it is the improvements onthat type of system which are of significance. The OLT does not usewavelength to control the focusing. It has a fixed focus and fixedwavelength laser. The quadrant photodetector for U.S. Pat. No. 4,883,352is only designed to compensate for lateral movement. One embodiment ofthe OLT has at least three (and preferably six) quadrant detectors whichwill allow for compensation of lateral, depth, and rotational (pitch,yaw, and roll) motions. It is also apparent that the U.S. Pat. No.4,883,352 system is also optimally designed for projection onto a flator effectively flat (as seen by the galvanometer scanner) objects. TheOLT, in contrast, is designed to project onto highly contoured parts,based upon information from a 3-D model of the projection pattern.

U.S. Pat. No. 4,818,098 relates to a projection system for flat patternto flat pattern projection. It uses multiple image position sensors fordetermining planar rotational and translational shifts. (See column 7,lines 20, et seq.) The summary also mentions the use of graphicsprocessor with means to receive a variety of graphics inputs formexternal sources and a vector generator to provide the desired scanpositions for the servos (column 8, lines 20, et seq.).

The present OLT, in contrast, is designed to project 3-D images oncontoured surfaces, rather than flat surfaces, as taught in U.S. Pat.No. 4,818,098. Planar projections are a degenerate condition for theOLT. The initial OLT had quadrant detector input to detect any relativemovement of the projection object with respect to the projector, and isdesigned to correct for these rotations and translations. The multiplesensors in U.S. Pat. No. 4,818,098 are clearly designed to compensateonly for planar rotations and transitions.

It is also apparent that the use of the graphics interface by U.S. Pat.No. 4,818,098 is designed to generate a flat pattern from another dataset. Again, the present OLT can be contrasted in that it does notgenerate an intermediate flat pattern in order to determine the commandsthat are sent to the galvanometers. The advantage of the present systemis that distortions which can be generated by applying 3-dimensionalrotations to flat pattern algorithms can be totally avoided.

SUMMARY OF THE INVENTION

This invention relates to a rapidly scanning laser system that utilizes3-D data sets and accurately identifies locations on an object. Therapidly scanning laser system is a laser spot which moves from locationto location with sufficient speed to appear as a continuous, butflickering, line. This rapidly scanning laser is used for locating pliesof material in the hand layup of composite parts and locating templatesor paint masks during the painting of aircraft. The present systemcomprises a controller, a laser projector, and a data set defining thepattern to be projected. Reference locations on the object have the samecoordinate system as the 3-D data set of the projected image. Accordingto one embodiment these reference locations can be fitted with activesensors (quadrant detectors) or preferably cooperative targets whichreturn the laser light back to the laser projector for detection, thusallowing the system to account for rotational and translationaldifferences between the projector and object being projected upon. Analgorithm is used to calculate the position and orientation of theprojector relative to the reference sensors. Given the 3-D data set andthe computed position of the projector, the horizontal and verticalangles corresponding to each point of the 3-D data set am calculated,and the projector is then sequentially pointed to each of the calculatedpositions. Various interpolation schemes smooth the motion of thegalvanometers and reduce the following error of the control system.Information on ply material and orientation can also be projected. Keyfeatures of the present system include: numerical algorithms thatcontrol the manner in which the 3-D set is projected onto contouredsurfaces and also computes the position of the laser projector relativeto the projection surface. Technology for determining the positions ofthe reference locations which include active targets (quadrantdetectors) on or adjacent to the projection surface, or cooperativetargets on or adjacent to the projector surface, which return the laserlight back to the laser projector by various schemes, thus allowing thestreamlined calculation to occur. A further embodiment of this inventionutilizes reference targets instead of the aforementioned active sensors(quadrant detectors).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the present laser projection systemshowing an exemplary reference sensor system; and,

FIG. 2 is a block diagram partially in circuit schematic of a referencesensor system coupled downstream from the reference sensors shown inFIG. 1 in accordance with a first embodiment of the invention;

FIG. 3 is a geometric diagram for derivation of OLT equations;

FIG. 4 shows an OLT projector system for obtaining measurements of thereturning laser beam;

FIG. 5. is an electronic block diagram for quadrant detection of thereturn laser beam;

FIG. 6 is an electronic block and schematic diagram for power detectorand cooperative target;

FIG. 7 is a further electronic schematic for power detector andcooperative target; and

FIG. 8 is an OLT projector system for the third embodiment using anacousto-optic deflector.

DETAILED DESCRIPTION OF A FIRST EMBODIMENT OF THE INVENTION

This invention relates to a rapidly scanned laser system that accuratelyidentifies locations on an object. (Rapidly scanned means a scannedlaser spot which moves from location to location with sufficient speedto appear as a continuous line.) Two exemplary applications for thisdevice are:

1. locating plies of material in the hand layup of composite parts.

2. locating templates or paint masks during the painting of an aircraft.

Another for a slowly scanned system is in identifying locations on anaircraft during assembly, e.g., locating stringer clips and shear tieson an aircraft during final assembly. (Slowly scanned means a stationarypointer which can be moved incrementally from point to point.)

Prior high speed scanning of laser light for the purpose of precisionlocation of features has not been satisfactory in precision applicationsfor many reasons, most notably:

Prior users have utilized algorithms requiring flat pattern projectionas opposed to directly projecting three-dimensional (3-D) data. Arequirement was that 3-D data first be converted into a flat pattern byassuming system constraints such as projector position and orientation;and second, introduction of a projection plane on which the flat patternis generated. Thus, the fiat pattern could only be accurately projectedfrom one position. In a realistic environment, it is necessary tosimplify and account for changes in an object's location and orientationwith respect to the laser projector. This is necessary to simplify theinitial alignment and subsequent adjustments for relative movementbetween the projector and target object. Algorithms which scale, skew,translate, and rotate flat patterns will not accommodate thisrequirement. A prior attempt was the installation of a translation stagein the projector head so that for a given application, the projectorsystem will allegedly be self-correcting. This will not provide afunctional projection system because translations alone will not correctfor rotations which will occur.

A laser projector comprises three major components, as shown in FIG. 1;and includes:

1. controller 10 (i.e., operator interface and projector driver);

2. a laser projector 20, having the data set defining the pattern toproject; and,

3. reference sensors 30 positioned on the object 40;

4. their arrangement return and data set (controller) system.

This invention addresses inadequacies in projection systems via aprocess which is specifically designed to directly use 3-D data, andwhich will allow the system to account for rotational and translationaldifferences between projector 20 on object 40 on which the laser lightis pointed. In order to do this, however, it is necessary to havereference sensors 30 on object 40 that the laser light is pointed.Sensors 30 must be in a known relationship to the 3-D data set to beprojected, i.e., reference sensors 30 should use the same coordinatesystem as the 3-D data set. FIG. 1 also shows target object 40 where the3-D data set lays (in this figure a layup mandrel) and reference sensors30 mounted on target object 40.

An abbreviated summary of the process includes the following steps:

1. Reference sensors 30 positions are measured (illuminated) withprojector 20. The horizontal and vertical angles of the two scanners arerecorded.

2. A computer algorithm is used to calculate the position andorientation of projector 20 relative to sensors 30 (see FIG. 2):##EQU1## Where Ch A and Ch C, and Ch B and Ch D are opposite sides ofthe quadrant reference sensor.

3. Given the 3-D data set and the computed position of projector 20, thehorizontal and vertical angles for each point of the 3-D data set arecalculated.

4. Projector 20 is sequentially pointed to each of the calculatedpositions. Information on ply material and orientation can also beprojected.

5. Periodically, reference sensors 30 positions are measured again toascertain if there has been relative movement between them and projector20. If movement is detected, steps 2, et seq., are executed again.

A key feature of the process is the numerical algorithm utilized insteps 2 and 3 infra. The equation solved is: ##EQU2## where: H ANDV=horizontal and vertical angles projected.

X, Y, and Z=the position of projector 20.

Omega, Phi, and Kappa=the angular orientation of projector 20.

x, y, and z=the position of locations to be projected.

m_(ij) =the angle cosines for the orientation of projector 20.

m₁₁ =cos(Phi) cos(Kappa)

m₁₂ =sin(Omega) sin(Phi) cos(Kappa)+cos(Omega) sin(Kappa)

m₁₃ =-cos(Omega) sin(Kappa cos(Kappa)+sin(Omega) sin(Kappa)

m₂₁ =-cos(Phi) sin(Kappa)

m₂₂ =sin(Omega) sin(Phi) sin(Kappa)+sin(Omega) cos(Kappa)

m₂₃ =cos(Omega) sin(Phi) sin(Kappa)+sin(Omega) cos(Kappa)

m₃₁ =sin(Phi)

m₃₂ =sin(Omega) cos(Phi)

m₃₃ =cos(Omega) cos(Phi)

The application of the above equation to step 2 is difficult because itsinverse function must be formed. In order to do this, a first orderTaylor series expansion for the variables X, Y, Z, Omega, Phi, and Kappais written. Estimates of the variables are used in the Taylor expansionand a least squares analysis iterates to improve the solution. For thismethod to work, at least three reference sensors 30 are required;however, for practical application, at least six sensors should be used.(Six sensors work well because it is best to spread the measurement andencoding errors over more than a minimum set of equations.)

The application of the algorithm to step 3 is straightforward. Theposition and orientation of projector 20 was computed in step 2, and theknown positions for projection are then applied to the equation and thehorizontal and vertical angles are computed.

The design of the reference sensors may feature area measuring devices,such as CCD arrays (i.e., area arrays), and quadrant detectors. As such,these devices can be digital, analog, or a combination thereof. FIG. 2shows an exemplary quadrant detector reference sensors system. Theschematic shown in FIG. 2 is for analog sensors 30, which are converteddownstream through multiplexer 50 and analog to digital converter 60 todigital signals 62. Algorithms in the computer at controller 10 performautomatic gain control (i.e., computations to reduce referencesensitivity to changes in laser power).

It is required that necessary reference sensors 30 be mounted on object40 for which the projected pattern is designed. A practical method whichcan be used for doing this is to mount reference sensors 30 on the toolin a repeatable manner and then measure that location with anothermeasuring system (e.g., computer-aided theodolites). The object (40) canalso be fabricated with mountings for reference sensors (30).

The locating of reference sensors 30 can be partially automated. Theoperator will need to manually drive projector 20 to each sensor 30.Once the laser beam strikes the cell, an automated control systemoverrides the manual motion and drive the projected laser light to thecenter of the reference sensor. In such an automated system, arotational transformation algorithm is required because the initialrotation of the sensor with respect to the orientation of the projectoris unknown. This simplifies the operation and increases the accuracy.

Distortion compensation (i.e., pin cushion) should be compensated.Distortion compensation is utilized when computing the angles forprojector 20. An inverse function for distortion is utilized whencomputing the location and orientation of the projector.

A CAD data file or measured data file is utilized to define the X, Y,and Z positions for laser projection. There are many optimal methodswhich may be utilized when recording this data for use in projecting theply outline.

These several exemplary methods include:

1. defining a constant velocity path around the circumference of theply.

2. defining points (e.g., equally spaced) around the periphery of theply.

3. defining point to simulate eyebrows around the part.

4. defining equally spaced points around the dircumference of the plywhose delay between each point is proportioned to the curvature at thatpoint.

The selected exemplary method being number 3. Such method limits theamount of data which needs to be projected, and thus allows the speedfor projection to increase, thereby reducing the apparent laser flicker.

DETAILED DESCRIPTION OF A SECOND EMBODIMENT OF THE INVENTION

The present invention relating to optical layup template (OLT) addressesinadequacies in existing projection systems designed to project plyinformation. Ply marking systems marketed are limited to two dimensionalpatterns, and are not capable of projection onto highly contouredsurfaces. Furthermore, severe restrictions on the placement of theprojector in these systems relative to the surface for projection makesthe system cumbersome (locating pins are needed) and tooling expensive(fabrication personnel need to control the surface locations withrespect to the underside of the tool where the locating pins seat).There is also a limit on the saving of factory labor because the use ofsailcloth templates allow the hand marking of multiple plies so thetemplate is not used for each addition of material to the layup.

The use of templates on highly contoured layup mandrels generallyinvolves more complex templates. These templates are often comprised ofan elastomeric material which is strengthened by a fiberglass substrate.Tooling is required to form these templates to the proper contour. Thusthe cost of manufacturing the highly contoured templates is more thanthe simple flat templates. In addition, the weight of the templates isgreater than that of sailcloth, thus making handling of the contouredtemplates more difficult. As a consequence, a projector to handle 3Dinformation is of great importance.

The OLT is designed to project three dimensional information onto asurface. It is comprised of three main components:

1. The OLT controller.

2. The OLT projector head.

3. The reference targets.

The hereinbefore described first embodiment OLT used quadrant detectorsrather than reference targets. The OLT controller according to thesecond embodiment hereinafter described, is designed to accept operatorinstructions, read data files containing xyz information and convertthat data into horizontal and vertical angles, projectable information,for the OLT projector head. The controller also takes information fromthe OLT projector head regarding angular positions of the referencetargets. This allows the controller to compute the position of theprojector head relative to the reference targets. This is the key to theOLT since knowledge of the relative positions allows all point positionson the surface to be easily calculated and projected.

The OLT projector head is comprised of galvanometer scanners which steera laser beam. Also included in the OLT projector are a modulator tocontrol laser beam intensity, a telescope to expand and focus the laserbeam, a beam splitter to separate a returning laser beam from theoutgoing beam and onto a detector, and electronics to detect thereturning laser beam.

The reference targets, or more generally, cooperative orretro-reflective targets are optical devices which have thecharacteristic of returning light in the same direction from whence itcame. These devices can be corner cubes, retro-reflective materials,chrome tooling balls, etc. (The use of a plain mirror would not be acooperative target because the light usually reflects off at a differentangle.) The cooperative targets are positioned at the periphery of thelayup surface so that all subsequent point computations for plypositions are "interpolations" from the positions of the referencetargets.

Three or more reference targets are required, although there arediminishing accuracy benefits when more than seven reference targets areused.

The OLT has two major advantage over the flat pattern system:

1. It projects 3D information onto a contoured surface thus eliminatingcostly CLTs.

2. It uses cooperative targets to measure the relative position betweenthe projector head and contoured surface and actively adjust the 3Dprojection to accommodate any relative movement, thus simplifying thetool process.

BACKGROUND

The OLT is not a measurement system. The OLT merely uses a laser toproject CAD data defining part outlines onto an LM (layup mandrel) orother structure. To do this, the relative position between the OLTprojector and LM must be known. Predefined reference positions are addedto the LM for this purpose. Once the rellative position is known, theCAD data for the outline is changed into horizontal and vertical anglesfor the OLT to project. It must be noted that the surface beingprojected upon is assumed to be accurate so the projected angles willcircumscribe the correct shape. A simplified process flow is as follows:

1. Load Data (X Y Z) for Reference Positions.

2. Record the Horizontal and Vertical Angles of the Galvanometers to theReference Positions.

3. Compute the Position (X Y Z Pitch Yaw Roll) for the OLT Projector

4. Load Data (X Y Z) for the Part Outline.

5. Compute the Horizontal and Vertical Angles for the Part Outline.

6. Project the Horizontal and Vertical Angles.

As can be seen, the reference positions are assumed to be correct.Angles to these positions are measured and a calculation determines theposition and orientation (X, Y, Z, Pitch, Yaw, Roll) of the OLTprojector.

SUMMARY OF COMPUTATION FOR 3D PROJECTION

The basic algorithm for computing the relationship between the projectorgalvanometers and projection surface is: ##EQU3## where: i,j,k arevectors from the internal angles of the projector head.

X_(p),Y_(p),Z_(p) are the coordinates for the position of the projectorhead.

x_(i),y_(i),z_(i) are the coordinates of a point on the surface to beprojected.

M is the angle cosine matrix.

FIG. 3 shows the geometric relationship of the above equation. From FIG.3 it is apparent that the relationship of the internal angles of theprojector to the i, j, k vector is a slightly modified sphericalcoordinate system:

k=d

j=d*tan(V) ##EQU4## where: H is the horizontal angle of thegalvanometer.

V is the vertical angle of the galvanometer.

e is the separation of the horizontal and vertical galvanometers.

d is the distance from the galvanometer to the surface being projectedupon.

The slight modification to the spherical coordinate system is in the iterm and accounts for the separation of the horizontal and verticalgalvanometers.

Three important things should be noted in these equations. First, all ofthe equations contain the factor d, which is unknown and must beremoved. Second, the separation of the mirrors in the galvanometer isaccounted for by the factor `e`. (This latter correction removes thedistortion mentioned in the first embodiment. Third, other optical andmechanical errors can be modeled and added to the i,j,k equations, forexample squareness of axes galvanometer shaft to mirror alignment.

To use these equations function, the position of the projector is solvedfirst. The equations are solved by dividing the i and j equations by k,and assuming initially that the separation e=0, thus eliminating thefactor `d`. The result are two non-linear equations:

    tan (v)=(m.sub.11 (X.sub.p -x.sub.i)+m.sub.12 (Y.sub.p -Y.sub.i)+m.sub.13 (Z.sub.p -z.sub.i))/(m.sub.31 (X.sub.p -x.sub.i)+m.sub.32 (Y.sub.p -y.sub.i)+m.sub.33 (Z.sub.p -z.sub.i))

    tan (H)/cos (V)=m.sub.21 (X.sub.p -x.sub.i)+m.sub.22 (Y.sub.p -y.sub.i)+m.sub.23 (Z.sub.p -z.sub.i))/(m.sub.31 (X.sub.p -x.sub.i)+m.sub.32 (Y.sub.p -y.sub.i)+m.sub.33 (Z.sub.p -z.sub.i))

These equations are linearized for the OLT projector positions and anglecosine matrices using a first order Taylor series expansion about anestimated answer. (See Appendix 1 for a C program listing of thegeneration of the partial derivatives.) The least squares methods forobtaining this solution are well documented in texts such as "Elementsof Photogrammetry" by Paul Wolf or the "Handbook of Photogrammetry"published by the American Society of Photogrammetry and Remote Sensing.

To obtain a meaningful solution, three or more reference targets must bemeasured by the OLT projector head. These then generate an overdetermined least squares solution noted above. This is apredictor--corrector type whose corrections are for the projectorposition and angle cosine matrices.

Once the solution for e=0 is obtained, the computed value for d andmeasured value for e can be added to the above equations. The constantterm in the second equation above changes from:

    tan (H)/cos (V)

to

    tan (H)*(e/d+1/cos (V)

Through each iteration the value for d is estimated and inserted intothe constant term of the solution by using the original definingequation (1):

    d=m.sub.31 (X.sub.p -x.sub.i)+m.sub.32 (Y.sub.p -y.sub.i)+m.sub.33 (Z.sub.p -z.sub.i)

When the correction term of the predictor--corrector get sufficientlysmall, the solution has been obtained.

The process of obtaining the angles corresponding to a given OLTprojector position and a set of xyz positions to project is straightforward:

V=arctan(i/k)

H=arctan(i/(e+sqrt(k² +j²))

where i,j,k are computed from the original equations, (1) above.

There are several geometric compensations which must be corrected, andthese have been modeled and included in the numerical methods for theOLT:

1. Linearity of galvanometers.

2. Squareness of axes.

3. Mirror to galvanometer shaft alignment.

The remaining geometric errors have been minimized by precisionfabrication of the OLT projector.

The linearity of the galvanometers is accounted for by a look up table.

SUMMARY OF USE OF COOPERATIVE TARGETS AND DESIGN OF OLT HEAD REQUIRED TOIMPLEMENT THAT USE

The initial design (first embodiment hereinbefore described) of the OLTused active targets (quadrant detectors) to measure the location of theOLT projector head. This required wiring from the OLT controller to theobject being projected upon. This is viewed as an impediment to theapplication of this technology since wires cause a tripping hazard andthey take time to install and use. Furthermore, an algorithm wasrequired to determine the rotation of the active targets with respect tothe motion of the galvanometers.

These drawbacks are corrected through the use of cooperative targets inaccordance with the second embodiment. (As stated previously, acooperative target is an optical device which has the characteristic ofreturning light in the same direction from whence it came. These devicescan be corner cubes, retro-reflective materials, chrome tooling balls,etc.) Cooperative targets eliminate the wiring from the controller tothe object being projected upon because the sensing devices are insidethe OLT projector. Algorithms to determine the rotation of the activetargets with respect to the motion of the galvanometers are not neededbecause the sensor is inside the OLT projector head and thus thegeometrical relationship between the two is known a priori.

Several schemes were devised for measuring the laser light returning tothe OLT projector. Three major schemes are:

1. The use of corner cubes and quadrant detector.

2. The use of cooperative targets with analog power signal returned fromthe OLT projector to the OLT controller.

3. The use of cooperative targets with digital power signal returnedfrom the OLT projector to the OLT controller.

The advantage of the first method is that it is simple to implement (andwas the first method demonstrated) because it has the most optical laserpower returned to the OLT projector head and because the corner cubedoes not disperse the laser beam. When a null reading between oppositecells is obtained for the signal on the quadrant detector the correctcenter of the corner cube has been located. Furthermore, using thecorner cube, the OLT controller can drive the galvanometers, with closedloop feedback, to the correct center of the corner cube. Drawbacks ofthis method are that the corner cubes tend to be large, bulky to mount,and their narrow acceptance angle makes them undesirable in amultiprojector system. If solid corner cubes are used there is also adisplacement error caused by refraction of light within the corner cubeif the corner cube is not pointed directly at the OLT projector.

The use of cooperative targets with an analog power signal returned tothe OLT controller is a more difficult implementation. Cooperativetargets tend to scatter the laser beam, thus making precise motionsmeasurements with a quadrant detector impossible. The vicinity of thecooperative target is raster scanned and the centroid for the powerreturn determines the correct center of the target. Advantages for thisscheme are that cooperative targets such as photogrammetric targets aresmall (often the size of a shirt button) and they (photogrammetrictargets) are wide angle so that adjacent projectors are able to use thesame targets. (It should be noted that a corner cube could also be usedas a cooperative target.) A potential disadvantage is that the locatingof the centroid is slower than the use of the quadrant detectors.

The use of cooperative targets with a digital power signal returned tothe OLT controller is also a more difficult implementation. Thedifference between this and the previous method is that a digitalsignal, rather than analog signal, is returned to the OLT controller. Acomparitor is used to convert the analog signal at the power detectorinto a one bit digital signal. The value of the comparitor can be set byeither a digital signal from the computer or be configured in theelectronic hardware. The comparitor circuit is also faster than ananalog to digital converter used in scheme 2 above. An advantage is thewide dynamic range for the analog to digital comparitor circuits. Apotential disadvantage is that finding the centroid of the cooperativetarget is limited by the speed of the raster scanning, and will beslower than using a quadrant detector.

FIG. 4 shows the two general hardware schemes for obtaining measurementsof the returning laser beam. The key to detection is the use of apellicle beam splitter in the laser beam path. This allows the OLTprojector to measure the position or power of the laser beam returned bythe cooperative target. (An off axis detector system could be used tocollect the returning light, however this scheme would not be effectiveif corner cubes are used since the returned beam is still collimated.)Another important feature to note is that the OLT is a fixed focussystem, whose focus is minimized at the average distance from the OLTprojector to the layup surface. If an active focusing device were used,such as a galvanometer driven lens system, there would be a slowerresponse time for focusing than for pointing the laser beam, thusslowing the projection speed.

From FIG. 4 it is evident that the laser is not aligned along theoptical axis of the telescope. The acoustic modulator, used to modulatethe laser beam itensity, is aligned so that the first order beam isaligned to the optical axis. This is a simple safety concern so that isthe acoustic modulator is not energized, there will be no lighttransmitted by the OLT. This scheme does not a power penalty in thatonly about 60% of the light is transferred to the first order beam.

DETAILS FOR SCHEME 1, SUPRA

To effect the scheme 1 design the beamsplitter in FIG. 4 returns thelaser beam to a quadrant detector. This scheme requires the use ofcorner cubes as the cooperative targets so that the return laser beam iscohesive and can be measured. FIG. 5 shows a diagram of the electronicswhich captures the return signal.

The quadrant detector is actually a two dimensional lateral effect cell.A standard quadrant detector could probably also be used. Notice thereis no focusing lens in front of the lateral effect cell. If a quadrantdetector is used, a focusing lens might be used to ensure that thereturn beam impinges on all of the quadrants of the detector. Fourtransimpedance amplifiers are used to convert the current from each ofthe detector quadrants to a voltage. Following this, analog to digitalconversion is performed. Note that four A/D converters are used. Thereason for this is to speed the A/D process. A single A/D converter andfour channel sample/hold could be used. The vertical and horizontalsignals for the return laser beam are then adjusted by an automatic gaincontrol algorithm which adjusts for variations of returned lightintensity:

    Vertical=(B-D)/(B+D)

    Horizontal=(A-C)/(A+C)

it is also obvious that a different types of signal processing can beperformed, e.g., averaging several measurement, to obtain a betterestimate of the vertical and horizontal measurements.

The closed loop correction for the center of the quadrant detector isperformed by noting that the relative position of the return beam andthe null center of the quadrant detector can be used to drive thegalvanometers toward the null center. It is important to remember thatthe angles which are deflected by the galvanometers translate intodifferent motions on the quadrant detector depending on the rangebetween the galvanometers and the quadrant detector. This fact can setthe maximum step size make by any move of the galvanometer. Also, thestep size can be actively adjusted during closed loop operation bycorrelating the step size taken with the measured change on the quadrantdetector and adjusting the step size accordingly.

DETAILS FOR SCHEME 2, SUPRA

FIG. 6 shows the electronics for the power detector and cooperativetarget. In general the value of R_(f) is large because of the power lossfrom the cooperative target and the limited amount of light acceptedback into the OLT head. The variable resistor, R_(s), is used to offsetthe measurement for the ambient light (off the first reflection of thebeam splitter). The value from the A/D converter is transmitted to thecomputer where additional signal processing is performed. The scheme forthe power detector requires the galvanometers to raster scan thecooperative target, measuring the power at all the locations. Thecentroid is computed from all the data which is above some threshold.Other power centering schemes can be imagined such as fitting the powerreturn to a paraboloid or other shape which takes into consideration thepower level as well as the scan position.

DETAILS FOR SCHEME 3, SUPRA

FIG. 7 shows the schematic for the power detection scheme whichincorporates the analog comparator prior to sending a one bit digitalsignal to the computer. The threshold level for the comparator isderived from another digital signal from the OLT controller. Again, theoffset into the transimpedance amplifier is adjusted to cancel theeffect of ambient light from the first pass of the laser beam throughthe beam splitter. This system has not been tested. The scheme for thepower detector requires the galvanometers to raster scan the cooperativetarget, measuring the power at all the locations. The centroid iscomputed from all the data which is above some threshold. Other powercenter schemes can be imagined such as fitting the power return to aparaboloid or other shape which takes into consideration the power levelas well as the scan position.

In the first embodiment, the general equations: ##EQU5## which relate tothe horizontal and vertical angles of the galvanometer scanners for theprojector (the left side of the equations) with the relative positionsof the reference sensors and position and orientation of the OpticalLayup Template (the right side of the equations). The equation has beenimproved to account for "pin cushion" distortion generated by theseparation of the two mirrors of the galvanometer. ##EQU6## FIG. 3 isthe sketch for these equations and also contains the definitions for thevariables.

DETAILED DESCRIPTION OF A THIRD EMBODIMENT OF THE INVENTION

The prior designs (first and second embodiments hereinbefore described)of the OLT had a preferred embodiment using a pair of galvanometersscanners to steer the location of the laser beam. This is often animpediment to the application of this technology since the slow responsetime of the galvanometers cause visible flicker of the projected laserlight.

These disadvantages are corrected through the use of an acousto-opticdeflectors. The preferred embodiment uses an orthogonal pair ofacousto-optic deflectors in accordance with the third embodiment, FIG.8. The orthogonal pair of acousto-optic deflectors 100 steer the laserbeam faster than the galvanometers, thereby eliminating most if not allof the flicker. The first of the pair of acousto-optic deflectors steersthe laser beam using the controlling horizontal angle. The second of thepair of acousto-optic deflectors steers the laser beam using thecontrolling horizontal angle. The use of the orthogonal pair ofacousto-optic deflectors requires that the beam splitter, 102, be placedbetween the laser source 101 and the orthogonal pair of acousto-opticdeflectors. The telescope (beam expander) 103 needs to have largeroptics than the first and second embodiment for accommodating thedeflected laser beam. The disadvantages in the use of an orthogonal pairof acousto-optic deflectors 100 is that current technology foracousto-optic deflectors will reduce the scam area.

APPENDIX

The following is a solving engine written in C for computing and loadingthe partial derivatives for solving the position of the OLT projectorhead and the orientation of angle cosine matrix. Note that thisderivation also contains information on adjusting for out of squarenessof the horizontal and vertical galvanometers. ##EQU7##

What is claimed is:
 1. A laser projector comprising in series opticalpath: a laser light source for producing a coherent beam of light;meansfor deflecting a reflected returned laser light out of the outgoingoptical path; a laser beam steering device for directing said laser beamat an object; a telescope for expanding and focusing said laser beam;and means for detecting said laser light returning through and deflectedout of said optical path.
 2. A laser projection system comprising:alaser light source for producing a coherent beam of light; a telescopefor expanding and focusing said laser beam; a laser beam steering devicefor directing said laser beam at an object; said laser beam steeringdevice comprises an acousto-optic deflector; controlling means forcontrolling said beam steering device and deriving the angles which saidbeam steering device must point; a plurality of reference targets, on oradjacent to said object for returning the laser light back to the saidbeam steering devices; means for deflecting the returned laser light outof the outgoing optical path; a single sensor for intercepting anddetecting said reflected laser beam; means connected to said sensor fordetecting the laser beam impinging on the surface of said sensor; meansfor determining the position and orientation of the beam steeringdevices with respect to said sensor; means for computing the anglesrequired for said object, said three-dimensional data in the samecoordinate system as said targets.
 3. A laser projection systemaccording to claim 2 wherein said acousto-optic deflector comprises anorthogonal pair of acousto-optic deflectors.